Discussion
In semi-arid alpine regions of the Tibetan Plateau, climatic warming and
increased precipitation are expected to cause widespread shrub
encroachment into alpine grassland ecosystems (Lu et al., 2022; Wang et
al., 2022).Shrubs encroaching will promote plant growth, increase
vegetation productivity, and lead to accumulation of soil carbon and
nutrients in water-limited alpine grasslands (Pistón et al., 2016; Cui
et al., 2023), and, in turn, results in the strengthen of ecosystem
functioning (Wang et al., 2023). Consistent with these findings, our
study also found that shrub encroachment enhanced ecosystem
multifunctionality index of the Tibetan semi-arid alpine steppes (Fig.
4a). Nevertheless, our finding also revealed divergent above- and
belowground ecosystem functioning responses to both Leguminosae and
Non-Leguminosae shrubs encroachment in semi-arid alpine steppes, with
the BEMF index rather than AEMF index increased following shrub
encroachment (Fig. 4b-e). Briefly, our results further suggest that
shrub encroachment had neutral effects on aboveground ecosystem
functions, specifically plant cover, species richness, and below-ground
plant biomass (Fig. 2a-e and Fig. 3). However, shrubs encroached into
semi-arid alpine steppes generally tend to enhance the belowground
ecosystem functions, that is, higher soil nutrients, and soil carbon and
nitrogen stocks (Fig. 2f-l and Fig.
3).
The encroaching shrubs of C. spinifera and D. fruticosaare generally drought tolerant and nutrient rich. These encroaching
shrubs can create a microhabitat favoring growth of herbaceous plants
through buffering of extreme soil temperatures and drought stress and
increasing in soil nutrients availability, contributing to the increase
of biodiversity and ecosystem functioning (Cui et al., 2023; Zhao et
al., 2023). However, species richness, diversity, and productivity of
herbaceous plants in the encroached areas also could be lower due to the
encroaching shrub species competing for soil nutrients and shading light
resources (Zhang et al., 2022). Therefore, the net impacts of shrub
encroaching on biodiversity and ecosystem functioning often depend
greatly on its facilitation and inhibition effects. In the present
study, we found that encroaching of both shrubs tended to increase
coverage of the herbaceous plants, but had neutral effects on the
diversity and production of herbaceous plants (Fig. 2 and Fig. 3). Thus,
the finding indicated that plant diversity and vegetation productivity
are an unlikely major mechanism for the shrubs encroaching induced
facilitation impacts on ecosystem functioning of the semi-arid alpine
steppe.
The availability of soil water and nutrients mainly limited plant growth
in alpine steppes (Sun et al., 2020; Cheng et al., 2022). With the
encroachment of shrubs, soil organic matter and nutrients gradually
accumulate in the soil, creating the ”fertile islands” effects of shrubs
(Valencia et al., 2015), which is conducive to the increase of BEMF and
EMF. In addition, shrub encroachment could enhance the soil water
availability under the shrub canopy by reducing evaporation under the
shrub canopy (Iyengar et al., 2017; Liu et al., 2020). It was noted that
shrub encroaching induced mitigation of soil water and nutrient
deficiencies was the major reason for the abrupt facilitation of
ecosystem functioning (Ale et al., 2023; Wang et al., 2023). However,
former studies indicated that soil nutrient rather than water
availability mainly mediated the encroaching of shrubs caused
facilitation impacts on the ecosystem functions of semi-arid alpine
grasslands (Cui et al., 2023; Zhao et al., 2023), which is also well
reflected by our finding that showed the increased EMF following shrub
encroachment (Fig. 4). In the present study, despite the significant
relationships between soil water content and species richness of
herbaceous plants were showed after encroachment by the tow shrubs, we
found that both encroaching shrubs had no significant effects on the SW
(Table 3), suggesting that water availability might not be the major
reason for the changes of ecosystem functioning of the semi-arid alpine
steppes.
Previously have been emphasized that the traits of encroaching shrubs
(e.g., Leguminosae or non-Leguminosae species) could affect the degree
of their effects on the ecosystem functioning (Eldridge et al., 2011).
In semi-arid alpine ecosystems, Leguminosae shrubs, compared with
non-Leguminosae shrubs, have great potential to alleviate nutrient
limitations and often to facilitate species richness, vegetation
productivity, soil microbial activities, and carbon accumulation as well
as ecosystem functioning under their canopies (Iyengar et al., 2017; Ale
et al., 2023). Our findings showed that the encroaching of both
Leguminosae and non-Leguminosae shrubs enhanced ecosystem functions of
semi-arid alpine steppes. Nevertheless, the encroachment of both
Leguminosae and non-Leguminosae shrubs had significant positive impacts
on the belowground ecosystem functioning rather than aboveground
ecosystem functioning in semi-arid alpine steppes. This may be because
the climate in this region is relatively dry, and shrub
encroachment-induced increase in soil nutrients may just slightly
promote the growth of herbaceous plants in the semi-arid alpine steppes.
In addition, in semi-arid region, most of these herbaceous plants with a
shallow root system might be more sensitive to water rather than
nutrients availability, and the shallow roots also makes them less
advantageous in competing for water and nutrient resources with the
deep-rooted Leguminosae shrub (Zhang et al., 2022). Such mechanisms also
could explain why there was a neutral response of the AEMF in semi-arid
alpine steppes after the encroaching of both shrub species.
Nevertheless, it is critical to note that our study was conducted
primarily in the water-limited region on the Tibetan Plateau, and these
semi-arid alpine steppes were highly responsive to changes in
precipitation and soil water conditions (Zhao et al., 2019). Our
findings based on observations highlight that the encroaching of both
leguminous and none-leguminous shrubs has the potential to substantially
strengthen ecosystem functioning of the semi-arid alpine steppes on the
Tibetan Plateau. These findings have important and broadly applicable
implications for developing management measures and better maintenance
of alpine ecosystems given the vast expanse of alpine steppes across the
Tibetan Plateau. Such changes to alpine grasslands ecosystem functioning
resulting from shrub encroachment should be adequately considered in
predictions of climate changes and anthropogenic activities impact
because they could challenge our understanding of how alpine grasslands
function.